A gyroscope is a sensor that measures rate or angle of rotation. Micromachined gyroscopes constitute one of the fastest growing segments of the microsensor market. The application domain of these devices is quickly expanding from automotive to aerospace, consumer applications, and personal navigation systems. A multitude of applications exist in the automotive sector including short-range navigation, anti-skid and safety systems, roll-over detection, next generation airbag and anti-lock brake systems. Consumer electronics applications include image stabilization in digital cameras, smart user interfaces in handhelds, gaming, and inertial pointing devices. Some applications require single-axis gyroscope (Z-axis) and some require multiple axis rotation sensing (about X and Y and/or Z axes).
Miniature gyroscopes can be used for navigation. Inertial navigation is the process of determining the position of a body in space by using the measurements provided by accelerometers and gyroscopes installed on the body. Inertial Measurement Units (IMU) for short-range navigation are vital components in aircraft, unmanned aerial vehicles, GPS augmented navigation and personal heading references. An IMU typically uses three accelerometers and three gyroscopes placed along their respective orthogonal sensitive axes to gather information about an object's direction and heading. The components of acceleration and rotation rate can consequently be interpreted to yield the object's accurate position in space. An IMU is self-contained and can perform accurate short-term navigation of a craft/object in the absence of global positioning system (GPS) assisted inertial navigation.
Current state-of-the-art micromachined vibrating gyroscopes operate at low frequencies (ω0=3-30 kHz) and rely on increased mass (M) and excitation amplitude (qdrive) to reduce the noise floor and improve bias stability. If operated in 1-10 mTorr vacuum, such devices can achieve quality factors (Q) values on the order of 50,000 mainly limited by thermoelastic damping in their flexures. It is known that the fundamental mechanical Brownian noise of a vibratory gyro is given by:
      Ω          z      ⁡              (        Brownian        )              ∝            1              q        drive              ⁢                            4          ⁢                      k            B                    ⁢          T                                      ω            0                    ⁢                      MQ                          Effect              -              Sense                                          where qdrive is the drive amplitude; ω0, M, and Qeffect-sense are the natural frequency, mass and effective quality factor at the sense mode, respectively; kB is the Boltzmann constant and T is the absolute temperature.
Current state of the art micro-machined gyroscopes operate at a relatively low frequency (5-30 kHz) in their flexural modes and have a Q of less than 50,000 in high vacuum which results in a high noise floor with limited mass. It would be desirable to reduce the noise floor of vibrating gyros without having to increase the mass and drive amplitude, which is difficult to achieve in low power and small size. As will be disclosed herein, a capacitive bulk acoustic wave gyroscope can accomplish this task by (1) increasing the resonant frequency by 2 to 3 orders of magnitude (to 2-8 MHz), and (2) increasing Q significantly by utilizing bulk acoustic modes that experience significantly less thermoelastic damping compared to flexural modes. The very high Q of the bulk acoustic modes will translate into superior bias stability in these gyros. Operation at high frequencies can increase the frequency bandwidth of the gyroscope by orders of magnitude, which decreases the response time of the sensors and relaxes the mode-matching requirements. Another benefit of increasing the resonant frequency of the gyro is in increasing the stiffness of the device by orders of magnitude, which translates into much higher shock resistance for the device (100 kG tolerance). In addition, the large stiffness of the device makes it less susceptible to air damping, which simplifies the packaging and reduces manufacturing cost by eliminating the need for high vacuum encapsulation.
U.S. patents relating to gyroscopes include: U.S. Pat. No. 5,450,751 issued to Putty, et al. entitled “Microstructure for vibratory gyroscope;” U.S. Pat. No. 6,128,954 issued to Jiang entitled “Spring for a resonance ring of an angular rate sensor;” U.S. Pat. No. 3,719,074 issued to Lynch entitled “Hemispherical Resonator Gyroscope;” U.S. Pat. No. 4,793,195 issued to Koning entitled “Vibrating cylinder gyroscope and method;” U.S. Pat. No. 6,848,304 issued to Geen entitled Six degree of freedom micromachined microsensors;” and U.S. Pat. No. 6,837,108 issued to Geen entitled “Micro-machined multi sensor providing 1-axis of acceleration sensing and 2-axes of angular rate sensing.”